1. Field of the Invention
The invention relates to a method of manufacturing a rotor that can be used in an electric motor or a generator, and more particularly, relates to a method of manufacturing a rotor by magnetizing separately a plurality of magnet materials embedded in a rotor core in a plurality of times.
2. Description of Related Art
Conventionally, as a rotor used in an electric motor or a generator, there is a rotor with a plurality of magnets that are embedded in a rotor core formed by laminating steel sheets. For example, such a rotor can be manufactured by embedding an unmagnetized magnet material in the rotor core and magnetizing the embedded magnet material.
The magnet material embedded in the rotor core is magnetized by applying a magnetic field from outside to the magnet material. Also, depending on an arrangement of a plurality of magnet materials in the rotor, sometimes it is difficult to magnetize them all by applying an external magnetic field once. In such a case, a method of magnetizing all the magnet materials by applying separately external magnetic fields that are different from each other a plurality of times is employed.
Further, when applying an external magnetic field to one rotor a plurality of times, the external magnetic field shall be generated in such a manner that the polarization of the magnet material that is magnetized by other external magnetic field is not opposite to the polarization of the final product. For example, it is difficult to manufacture a final product by polarizing reversely a magnet that has been polarized in the opposite direction by an external magnetic field applied previously.
For example, in Japanese Patent Application Publication No. 6-38459 (JP 6-38459 A), it is described a method of using a magnetization apparatus having a first winding portion and a second winding portion with a number of turns less than the first winding portion that are arranged alternately along outer periphery of a rotor to magnetize a plurality of sheets to be magnetized of the rotor. Also, in JP 6-38459 A, a magnetic field is generated by energizing the first winding portion and the second winding portion synchronously two times. That is, in the energizing of the first time, a portion of the sheets to be magnetized is magnetized, and in the energizing of the second time after the rotor is rotated, the sheets to be magnetized that are not magnetized targets in the first time are magnetized. Further, the first winding portion is a winding portion for magnetizing the sheets to be magnetized by the generated magnetic field. The second winding portion is a winding portion for generating such a magnetic field that the magnetic field generated by the first winding portion in the first and second magnetizations is not applied as a reverse magnetic field to the sheets to be magnetized that are not magnetized targets of the second magnetization. Thus, it is possible to improve the magnetization efficiency.
However, in the above prior art, the first winding portion for generating a magnetic field for magnetizing and the second winding portion with the number of turns less than the first winding portion are provided in different current paths respectively and are connected in parallel. Therefore, in order to completely prevent the sheets to be magnetized that are not magnetized targets from being polarized reversely by the magnetic field for magnetizing with respect to the polarization of the final product, it is necessary to generate the magnetic field with the first and second winding portions in a completely matched timing. This is because when the magnetic field is not generated by the second winding portion, there is a possibility that the magnetic field for magnetizing that is generated by the first winding portion is applied as a reverse magnetic field to the sheets to be magnetized that are not magnetized targets.
However, in order to generate the magnetic fields with the first and second winding portions in the completely matched timing, it is necessary to adjust the timing of starting the energizing of the current paths, the energizing time, the values of currents flowing in the current paths and the like with a very high precision. For this reason, there is a problem that it is very difficult to completely prevent the sheets to be magnetized that are not magnetized targets from being polarized reversely by the magnetic fields for magnetizing.